I am unable to run any commands against my remote repository at Visual Studio Team Services (VSTS) because authentication fails.
I am able to do pulls, etc. through
When using Atlassian SourceTree you need the basic credentials setup in Visual Studio Team Services (VSTS) like in the accepted answer.
Then in SourceTree go to menu Tools → Options → Authentication tab.
You need to set the non-email username that you set up as basic username credentials for the XXX.visualstudio.com host.
You will then be prompted for the password when accessing that VSTS Git repository.